1. Field of the Invention
The present invention relates to a semiconductor laser, and more specifically to a semiconductor laser having a plurality of unit structures stacked in sequence, each unit structure including an active layer sandwiched between clad layers.
2. Description of the Prior Art
A typical semiconductor laser has a unit structure in which an active layer is sandwiched between clad layers.
In order to obtain a stronger laser beam, a prior art technique has proposed to stack a plurality of such unit structures in sequence to form a semiconductor laser. FIG. 5 shows a section of such a semiconductor laser having two unit structures stacked one on the other.
In FIG. 5, the semiconductor laser comprises in sequence:
an n-GaAs substrate 61 having a thickness of about 100 .mu.m and a carrier density of 1 to 3.times.10.sup.18 cm.sup.-3 ;
a first n-Al.sub.0.5 Ga.sub.0.5 As clad layer 62 having a thickness of 1.0 to 1.5 .mu.m and a carrier density of 0.5 to 1.times.10.sup.18 cm.sup.-3 ;
a first undoped GaAs active layer 63 having a thickness of 0.01 to 0.1 .mu.m;
a second p-Al.sub.0.5 Ga.sub.0.5 As clad layer 64 having a thickness of 1.0 to 1.5 .mu.m and a carrier density of 0.5 to 1.times.10.sup.18 cm.sup.-3 ;
a third n-Al.sub.0.5 Ga.sub.0.5 As clad layer 65 having a thickness of 1.0 to 1.5 .mu.m and a carrier density of 0.5 to 1.times.10.sup.18 cm.sup.-3 ;
a second undoped GaAs active layer 66 having a thickness of 0.01 to 0.1 .mu.m;
a fourth p-Al.sub.0.5 Ga.sub.0.5 As clad layer 67 having a thickness of 1.0 to 1.5 .mu.m and a carrier density of 0.5 to 1.times.10.sup.18 cm.sup.-3 ; and
a p.sup.+ GaAs contact layer 68 having a thickness of 0.3 to 1.0 .mu.m and a carrier density of 1 to 10.times.10.sup.18 cm.sup.-3.
The semiconductor laser further comprises a p-side electrode 72 and an n-side electrode 71.
The semiconductor laser as constructed above can radiate a stronger laser beam because of provision of the first active layer 63 and the second active layer 66 in a single semiconductor laser.
When voltage is applied across the p-side electrode 72 and the n-side electrode 71 in the above prior art semiconductor laser in such a manner that the p-side electrode 72 becomes at a positive potential with respect to the n-side electrode 71, a p-n junction grown at the boundary between the second p-type clad layer 64 and the third n-type clad layer 65 is biased in the reverse direction. Consequently, the breakdown voltage of the p-n junction becomes above 10 V, resulting in substantially increased power consumption.
To avoid the reverse bias applied across the p-type layer 64 and the n-type layer 65, a prior art technique has been proposed to grow a tunnel diode structure between the p-type layer 64 and the n-type layer 65. More specifically, a thin p-type junction layer and a thin n-type junction layer are formed in sequence on the upper surface of the p-type layer 64, and the n-type layer 65 is formed on the n-type junction layer. With this construction, the thin p-type junction layer and the thin n-type junction layer are effective to grow a tunnel diode structure which prevents development of the reverse bias.
This technique, however, requires an additional process to grow the tunnel diode structure.